Throw-away tipped drill bit

Abstract

A drill bit has a throw-away drill bit tip in the form of an insert (21, 31) secured to a shank (22, 32) to be mounted in a drilling tool. The insert (21, 31) is disengageably mechanically connected to the shank (22, 32) without any screws. The insert (21, 31) is formed of a silicon carbide whisker reinforced alumina sintered body, and the shank (22, 32) is made of steel. Due to this structure, the insert (21, 31) has an excellent wear resistance and an adhesion resistance. The shank (22, 32) has a sufficient toughness against breakage. Such a drill bit does not require any resharpening for continuous use during bit life.

What is claimed is: 
     
       1. A drill bit having a longitudinal central rotational axis, comprising a shank made of steel, a throw-away insert forming a drill bit tip made of an alumina sintered body reinforced by silicon carbide whiskers, and mounting means for releasably securing said throw-away insert to a forward end of said steel shank, said mounting means comprising holding portions arranged symmetrically to said central rotational axis, means for enabling at least one of said holding portions to elastically yield radially relative to said central rotational axis for elastically holding said throw-away insert with a frictional force fit in an axial direction, said steel shank comprising two axially extending shank sections contacting said throw-away insert for taking up torque moments, whereby said throw-away insert is held in a self-grip manner without any screw. 
     
     
       2. The drill bit in accordance with claim 1, wherein said silicon carbide whisker reinforced alumina sintered body contains 3.0 to 40 volume percent of silicon carbide whiskers having an aspect ratio within the range of 3 to 200 of whisker length to whisker width, the remainder being alumina. 
     
     
       3. The drill bit in accordance with claim 1, wherein said silicon carbide whisker reinforced alumina sintered body further contains a member selected from the group consisting of a carbide, a nitride, a carbonitride, a carbonized oxide, and an oxide of titanium. 
     
     
       4. The drill bit in accordance with claim 1, wherein said silicon whisker reinforced alumina sintered body contains ZrO 2 . 
     
     
       5. The drill bit in accordance with claim 1, wherein said silicon carbide whisker reinforced alumina sintered body contains a member selected from the group consisting of MgO, NiO, and Y 2  O 3 . 
     
     
       6. The drill bit in accordance with claim 1, wherein said steel forming said shank (22, 32) is selected from the group consisting of alloy tool steels (SKS) and chromium molybdenum steels (CMS). 
     
     
       7. The drill bit in accordance with claim 1, wherein said holding portions are part of said insert (21, 31) and of said shank (22, 32) to be engaged with each other respectively, and whereby said means for enabling comprise at least one slit (34) in at least one of said insert (21, 31) and said shank, so that opposite surfaces of said slit (34) are relatively moved when the respective holding portion is elastically deformed upon engagement to cause an elastic gripping force, thereby fixing said insert (21) to said shank (22, 32) without any screws. 
     
     
       8. The drill bit in accordance with claim 1, wherein said holding portions comprise a held portion (31a) forming part of said insert (21, 31) and two holding portions (33a, 33b) forming part of said shank (22, 32) for engaging left and right side surfaces of said held portion (31a), wherein said left and right side surfaces of said held portion (31a) and opposite holding surfaces of said holding protions (33a, 33b) are tapered, and wherein said surfaces are tapered at such angles that said engaging surfaces are brought into surface contact with each other with a press-fit upon engagement of said insert (21, 31) and said shank (22, 32) for forming said frictional force fit.